1. Technical Field
The present invention relates to a plug-in hybrid electric vehicle and a method of control.
2. Background Art
Plug-in hybrid electric vehicles (PHEV) are an extension of existing hybrid electric vehicle (HEV) technology, in which an internal combustion engine is supplemented by an electric battery and an electric machine to obtain increased fuel mileage and reduced vehicle combustion gas emissions. A PHEV has a larger capacity battery than a conventional HEV. A PHEV has the capability to recharge the battery from an external electric grid to decrease fuel consumption and to improve the vehicle's fuel economy in both a fuel/electric blended driving mode and an electric driving mode.
Conventional HEVs buffer fuel energy and recover kinematic energy in electric form to improve the overall vehicle system operating efficiency. Fuel is the principal energy source. For PHEVs, there is an additional source of energy—the amount of electric energy stored in the battery from the grid after each battery charge event.
While most conventional HEVs are operated to maintain a battery state of charge (SOC) around a constant charge level, PHEVs use as much pre-saved battery electric energy as possible before the next battery charge event; i.e. the relatively low cost grid supplied electric energy is expected to be fully used for propulsion and other vehicle functions after each charge. After the battery SOC decreases to a low conservative level during a given driving event, the PHEV resumes operation as a conventional HEV in a so-called charge sustaining mode.
To this end, two basic operating modes for a PHEV include a charge depleting (CD) mode and a charge sustaining (CS) mode. During a first travel distance after a charge, the fully/partially charged PHEV is driven in CD mode, where primarily the battery electric energy is used to propel the vehicle, gradually depleting the battery's electric energy. Once the battery SOC decreases to a predefined charge sustaining SOC level, the vehicle switches to CS mode, where the battery SOC is kept within a certain range around the charge sustaining SOC level and the vehicle is mainly powered, for example, by a gasoline engine (fuel energy), as is done in a conventional HEV.
The CD range is the distance a fully charged PHEV can travel in CD mode before the energy utilization pattern switches to the CS mode. By primarily using the battery electric energy to propel the vehicle, the fuel consumption will be minimized (blended CD mode). The vehicle may even operate with no gasoline fuel cost (all-electric CD mode), especially when the trip distance is less than or close to the CD range (˜30-60 miles in a typical design in multiple driving cycles). This control strategy is called base PHEV charge depletion strategy.
PHEVs desire to use as much pre-saved grid energy as possible before arriving at the destination; i.e., the grid supplied electric energy is expected to be totally utilized for propulsion. In some applications, however, a driver may like to save a certain amount of battery electric power for purposes other than for driving power.
The fuel economy of a PHEV can be optimized if the battery usage is adapted for the exact distance that the vehicle will be driven until the next charge. A standard approach has been to use a fixed (default) distance.